Existing couplings such as ball-couplings have the advantage of ease of attachment because the cup self-centres as it settles onto the spherical top of the tow-ball. However the range of articulation is restricted by the neck of the tow-ball contacting the edges of the cup.
Ball-couplings are retained on a tow-ball by a single tongue engaging the underside of the head of the tow-ball. To ensure automatic primary-locking the tongue is held in the engaged position by a powerful spring. In many jurisdictions a means of automatic secondary-locking of the tongue, in the position that precludes coupling separation, is mandated. Considerable force is required to retract the tongue, against the influence of the primary-locking spring, to a position where the ball-coupling's cup can disengage the tow-ball.
A variety of mechanisms have been adopted to fulfill the objective of providing a hitch, between a vehicle and a trailer, that will provide a greater range of articulation than is possible with a conventional tow-ball coupling. These mechanisms include:    (1) The pintle hook;    (2) An improved ring-type (pintle hook) coupling called the VC Coupling (Manufactured by Vehicle Components Australia);    (3) A ball-coupling with a tongue locking mechanism where secondary axes of rotation are provided between the cup, that engages a conventional tow-ball, and the trailer. The parts involved in the secondary axes of rotation dictate a design of the tongue locking mechanism that makes disengagement of the tongue particularly difficult;    (4) Couplings that have in common the manual insertion of a hitching-pin which links the vehicle and trailer components of the hitch together and thereby also provides one of three axes of rotation for movement of the trailer relative to the vehicle; and    (5) A coupling that self centres over a conical pillar and is retained on the pillar by a plate engaging in a circumferential groove in the pillar.
The pintle hook coupling is easy to connect but noisy and subject to wear because of the large amount of play between the ring and the hook. It is still used on heavy vehicles but has not found favour with domestic users wanting to use a car or four-wheel-drive to tow a trailer into rugged terrain.
The improved VC coupling pintle hook reduces noise but linking the vehicle and the trailer requires a special spanner to do up a clamp-bolt and the manual application of a clamp-ring-assembly to stop the clamp-bolt working loose. The range of articulation is less than that provided by other alternatives and the lack of automatic primary-locking and secondary-locking, against coupling disengagement, is a significant disadvantage.
The addition of supplementary axes of rotation to a ball-coupling with a tongue locking mechanism has been associated with the cup binding on the tow-ball and either damaging it or undoing it from the tow-bar. The mechanism has also been associated with difficulties disengaging the tongue that locks the cup onto a conventional tow-ball because the handle used to withdraw the tongue from the locked position is not aligned with the shaft that withdraws the tongue.
Couplings that employ hitching-pins suffer from the disadvantage of difficulty with inserting the hitching-pin when the respective vehicle and trailer components are not accurately aligned. Alignment of the components requires an exact height match between the vehicle part and the trailer part. Every time a vehicle moves its tow-bar subtly it changes height as power and/or brakes are applied. For this reason it is impractical to back the vehicle to a position where the trailer and vehicle components are engaged and ready to receive the hitching pin that definitively links them together. Instead the trailer has to be manually manoeuvered, with jockey wheel changes as required, until the vehicle and trailer components are fully aligned.
For this reason hitching-pin couplings are not particularly suitable for heavier trailers or caravans that cannot be manhandled into an exact position for successful insertion of the hitching-pin. Problems manoeuvering a trailer to obtain accurate component-alignment for hitching-pin insertion are multiplied if the wheels of the trailer are on uneven ground or in sand. Dual axle trailers are also hard to manually align because movement, other than directly fore and aft, requires sufficient force to scrub the tyres.
Hitching-pin removal can also be very difficult or impossible when the coupling is under load. The substantial lifting force of a jockey wheel, which can be used to separate a ball-coupling, is of no assistance in removing a wedged in hitching-pin.
Couplings that employ hitching-pins do not incorporate automatic primary-locking against coupling disengagement. This is because any mechanism that automatically delivered and locked the long hitching-pin into position would be too cumbersome for practical use. It would also be difficult to manually disengage any automatic hitching pin insertion mechanism that was forceful enough to insert the hitching-pin when the vehicle and the trailer components were not precisely aligned.
Some couplings, that employ hitching-pins, do offer an automated mechanism for secondary locking of the hitching-pin into a position that avoids coupling disengagement but this does not offer a requisite level of safety.
Couplings that self centre over a conical pillar do not have an automatic primary locking mechanism. In addition the primary locking mechanism of the plate engaging the circumferential groove is exposed to contamination with dirt that may impair functionality. The bearing surface of the conical pillar is likewise exposed to contamination with dirt that may seize the trailer component of the coupling onto the pillar.
Furthermore, couplings with a long vertical dimension impede full opening of any door on the rear of a vehicle.
The recently released “Drop on 35” coupling manufactured by Vehicle Components Australia does not provide automatic primary locking. This carries the risk of forgetting to lock the coupling before moving off. There is also a risk that an inquisitive person can “play” with the coupling while the owner is absent and inadvertently leave it in the unlocked configuration. Few owners would routinely check a coupling after a break in their travels.